Previous studies on modeling Johne's disease assumed that Map-infection on a dairy farm is only spread from infectious adults (>2 yrs) to calves. Preliminary inspection of a longitudinal dataset on full herd culture data casts doubt on this assumption. The Map-prevalence did decline substantially but did not fall to zero on a Pennsylvania dairy farm that had been performing (i) a Johne's test-and-cull program and (ii) very stringent management practices for a period of 20 years. Calf-to-calf transmission might be of substantial influence in maintaining Map-infections on low-prevalence farms. This is in agreement with experimental findings that Map-infected calves can (intermittently) shed low amounts of bacteria in their feces.It is therefore important to find out exactly which calves in a (low-prevalence) herd initially start spreading the disease to their herdmates, i.e. which calves become infected with Map as a result of vertical or horizontal cow-to-calf transmission. In this study we quantitatively estimated parameters on the vertical transmission of Map. The Pennsylvania dataset provides detailed information on pedigrees, calving dates, lactations, culling and sampling results (serology/ fecal culture) of all animals in the herd over the last 20 years. The dataset allows us to look at possible Map-transmission within a dam-daughter relationship, but also at Map-transmission within a given time frame, for instance as a result of temporarily contaminated calving pens.Quantitative estimates of dam to calf transmission will be presented and discussed in light of within-herd transmission dynamics. High quality longitudinal data are essential for further progress in understanding herd infection dynamics.